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Pilot-scale conversion of lime-treated wheat straw into bioethanol: qualityassessment of bioethanol and valorization of side streams by anaerobic
digestion and combustion
Biotechnology for Biofuels 2008, 1:14 doi:10.1186/1754-6834-1-14
Ronald HW Maas ([email protected])Robert R Bakker ([email protected])
Arjen R Boersma ([email protected])Iemke Bisschops ([email protected])
Jan R Pels ([email protected])Ed de Jong ([email protected])
Ruud A Weusthuis ([email protected])Hans Reith ([email protected])
ISSN 1754-6834
Article type Research
Submission date 15 November 2007
Acceptance date 12 August 2008
Publication date 12 August 2008
Article URL http://www.biotechnologyforbiofuels.com/content/1/1/14
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1
Pilot-scale conversion of lime-treated wheat straw into bioethanol: quality
assessment of bioethanol and valorization of side streams by anaerobic
digestion and combustion
Ronald HW Maas1§
, Robert R Bakker*1
, Arjen R Boersma2, Iemke Bisschops
3, Jan R Pels
2,
Ed de Jong1#
, Ruud A Weusthuis1 and Hans Reith
2
1Agrotechnology and Food Sciences Group, Wageningen University and Research Centre, PO
Box 17, 6700 AA Wageningen, The Netherlands. 2
Energy Research Centre of The
Netherlands, Biomass, Coal and Environmental Research, PO Box 1, 1755 ZG Petten, The
Netherlands. 3
Lettinga Associates Foundation, PO Box 500, 6700 AM Wageningen, The
Netherlands. §
Present address: Nobilon Bacteriological R&D, P.O. Box 320, 5830 AH
Boxmeer, The Netherlands. #
Present address: Avantium Technologies BV, Zekeringstraat,
1014 BV Amsterdam, The Netherlands.
RRB*: [email protected]
*Corresponding author
2
Abstract
Introduction
The limited availability of fossil fuel sources, worldwide rising energy demands and
anticipated climate changes attributed to an increase of greenhouse gasses are important
driving forces for finding alternative energy sources. One approach to meeting the increasing
energy demands and reduction of greenhouse gas emissions is by large-scale substitution of
petrochemically derived transport fuels by the use of carbon dioxide-neutral biofuels, such as
ethanol derived from lignocellulosic material.
Results
This paper describes an integrated pilot-scale process where lime-treated wheat straw with a
high dry-matter content (around 35% by weight) is converted to ethanol via simultaneous
saccharification and fermentation by commercial hydrolytic enzymes and bakers’ yeast
(Saccharomyces cerevisiae). After 53 hours of incubation, an ethanol concentration of
21.4 g/liter was detected, corresponding to a 48% glucan-to-ethanol conversion of the
theoretical maximum. The xylan fraction remained mostly in the soluble oligomeric form
(52%) in the fermentation broth, probably due to the inability of this yeast to convert
pentoses. A preliminary assessment of the distilled ethanol quality showed that it meets
transportation ethanol fuel specifications. The distillation residue, which contained non-
hydrolysable and non-fermentable (in)organic compounds, was divided into a liquid and solid
fraction. The liquid fraction served as substrate for the production of biogas (methane),
whereas the solid fraction functioned as fuel for thermal conversion (combustion), yielding
thermal energy, which can be used for heat and power generation.
Conclusions
Based on the achieved experimental values, 16.7 kg of pretreated wheat straw could be
converted to 1.7 kg of ethanol, 1.1 kg of methane, 4.1 kg of carbon dioxide, around 3.4 kg of
3
compost and 6.6 kg of lignin-rich residue. The higher heating value of the lignin-rich residue
was 13.4 MJ thermal energy per kilogram (dry basis).
Introduction
The limited availability of oil reserves and growing worldwide energy demands have resulted
in increasing energy prices. Furthermore, the utilization of fossil fuels has negative impacts
such as air pollution and the generation of the greenhouse gas carbon dioxide (CO2), which is
presumed to be one of the main anthropogenic contributors to the global warming effect.
These factors stimulate the exploitation of alternative renewable energy sources such as
biomass [1]. According to the Kyoto protocols, many of the industrialized nations need to
reduce their CO2 emissions by around 5% by 2010 as compared with the 1990 level, while a
further decrease will be compulsory in the long term [2]. One strategy to meet the global
increasing energy demand and the reduction of CO2 levels is the substitution of
petrochemically derived transport fuels by CO2-neutral biofuels such as ethanol [3].
Bioethanol can be obtained by the microbial conversion of carbohydrates derived from
biomass feedstocks such as agro-industrial residues [4-7]. Lignocellulose containing
feedstocks are widely available, relatively inexpensive, non-competitive with food
applications, sustainable in terms of CO2 emissions and, therefore, of potential interest for the
large-scale production of bioethanol. Lignocellulose consists of a complex fibrous structure of
polymeric sugars such as (hemi-)cellulose embedded in a matrix of the aromatic polymer
lignin [4, 8]. Wheat straw has been studied often as a raw material for microbial ethanol
production processes [9-11]. The conventional lignocellulose-to-ethanol conversion consists
of: (1) a pretreatment step; (2) a hydrolysis step; and (3) a fermentation step. Various physical
and chemical pretreatments have been developed to alter the structure of lignocellulosic
4
substrates [8]. An example is lime (Ca(OH)2) at mild temperatures (lower than 100°C), which
enhances the accessibility of (hemi-)cellulose for the subsequent enzymatic hydrolysis [12-
14]. We selected lime pretreatment as a model for further study as this has gained industrial
interest because of its perceived advantages over other pretreatment methods, including the
use of a low-cost chemical (lime) that is already in use in many agriculture-based, crop
processing schemes (for example, sugar refining), lower reactor investment costs, as well as
limited potential for the formation of degradation products. The hydrolysis, which is often
performed by a mixture of cellulolytic and hemi-cellulolytic activities, results in a hydrolysate
containing mainly soluble monosaccharides. Throughout the subsequent fermentation, these
monomeric sugars can be converted into ethanol by yeasts or bacteria with a high productivity
and efficiency. Alternatively, enzymatic saccharification and fermentation are performed
simultaneously at compromising reaction conditions in one reactor. So far only a limited
number of reports have been presented on the effects of lime pretreatment on the
simultaneous saccharification and fermentation (SSF) of lignocellulosic biomass, including
ethanol yield and the quality and valorization of side streams. Chang et al [15] submitted
lime-pretreated switchgrass, corn stover and willow wood to laboratory-scale SSF and
concluded that cellulose-derived glucose was extensively used by yeast to form ethanol
during SSF, and that the pretreatment did not result in a significant inhibition of fermentation.
In a more recent study, we investigated the use of lime-pretreated wheat straw for
simultaneous hydrolysis and fermentation to lactic acid by an enzyme preparation and
Bacillus coagulans DSM 2314 [16].
In a related study on the conversion of washed lime-treated wheat straw (LTWS) into ethanol,
the optimal reaction conditions were determined on a laboratory scale [14]. The objective of
the present paper is to study the effect of up-scaling on the performance of the lignocellulose-
5
to-ethanol SSF process, quality assessment of the bioethanol and valorization of the by-
product streams.
In the present study, ethanol was recovered from the fermentation broth via a distillation
process and its composition was analyzed to assess its suitability as a transport fuel. The
remaining distillation residue, consisting of water, lignin, non-hydrolyzed and non-fermented
organic components, minerals from the feedstock, the added process chemicals and other non-
ethanol fermentation products, was separated into a solid and liquid fraction. The combustion
behavior of the solid fraction, which represents a significant part of the energy input, was
studied and tested using a bench-scale experimental fluidized bed combustor (FBC) to assess
the suitability of the solid fraction as fuel for combined heat and power generation. The liquid
fraction, containing soluble organic components, was tested for its suitability for anaerobic
biodegradation yielding biogas (methane). Utilization options for the ashes derived from
combustion and sludge derived from anaerobic digestion are briefly discussed. This paper
represents, to the best of the authors’ knowledge, the first integral assessment of the
conversion of lignocellulosic material into bioethanol including the valorization of side
streams.
Results
Conversion of LTWS into ethanol by SSF
After chemical treatment, the LTWS suspension was washed and dewatered as described
previously [14]. The pretreatment was performed without significant loss of valuable
fermentable sugars. The resulting biomass had a dry-matter content of approximately 35% by
weight and contained 0.1 g/liter acetic acid. The polysaccharide composition of the
washed/dewatered LTWS was (by weight) glucan 37.2%, xylan 20.1%, arabinan 2.8%,
6
galactan 0.4%, rhamnan 0.2% and mannan 0.2%, whereas the remaining mass was primarily
lignin 21.4%, uronic acids 1.6% and ash 12.6%. The polysaccharide fraction, glucan and
xylan, accounted for 94% of the total polymeric sugars present in the LTWS. Therefore, we
focused mainly on the conversion of these polysaccharides into fermentable sugars and
ethanol via SSF in a 100-liter fermenter. The SSF process consisted of two phases, a
prehydrolysis phase and an SSF phase (Figure 1), both controlled at 37°C and pH 5.0. The
prehydrolysis phase aimed to reduce the viscosity (liquefaction) by adding LTWS manually
together with the commercial enzyme preparation GC 220 containing hydrolytic enzymes. A
total of 48.4 kg LTWS (16.7 kg dry matter) was added to the reactor during 6 hours of the
prehydrolysis phase. Another 2 hours of prehydrolysis (total of 8 hours) resulted in a glucose,
xylose and arabinose concentration of 15.4, 4.2 and 2.5 g/liter, respectively (Figure 1A). As
soon as the bakers’ yeast (Saccharomyces cerevisiae) was added to the reactor, the
accumulated glucose was rapidly fermented to ethanol and carbon dioxide (Figure 1B and C).
The wild-type bakers’ yeast only converts glucose and the pentose sugars, xylose and
arabinose remain present in significant amounts. After 24 hours of incubation, the glucose
concentration was low (less than 3 g/liter), suggesting that the enzymatic hydrolysis was the
rate-limiting step. After 48 hours of incubation, the ethanol concentration increased to
21.4 g/liter corresponding to 48% of the theoretical maximum glucan-to-ethanol conversion.
Based on this concentration, we concluded that 71% of the glucose monomers, liberated by
hydrolysis, were converted into ethanol. This value is close to ±80%, which is the theoretical
maximum conversion of glucose by yeast under anaerobic growing conditions [17]. At the
end of the SSF, a sudden production of lactic acid was observed, most probably as a result of
microbial contamination by, for example, lactic acid bacteria (Figure 1C). During the SSF
phase, the concentration of xylose increased to 11.8 g/liter, whereas the arabinose
concentration remained constant at 3.3 g/liter (Figure 1A). The efficiency of the enzymatic
7
saccharification and fermentation and the fate of the polymeric glucan and xylan after
53 hours of incubation are presented in Table 1. Part of the insoluble glucan initially present
in the LTWS was still present as polysaccharide (25%), whereas minor amounts were present
as oligomeric (6%) and monomeric sugars (1%). The largest part of the glucan was
hydrolyzed and converted via glucose into fermentation products, such as ethanol (and CO2)
(48%) and glycerol (5%), whereas the remaining part (15%) was most likely used mainly for
yeast growth. The situation was different with xylan, which remained present as
polysaccharide (26%), oligomeric sugars (52%) and xylose (24%).
Quality of ethanol
To determine the quality of the produced ethanol for application as a transportation fuel,
ethanol was distilled from the fermentation broth. The distillate contained an ethanol
concentration of 11.5% by weight. The ethanol content was upgraded to approximately 90%
by weight in a second distillation step. The distillate was subsequently analyzed for potential
contaminants (Table 2). These results showed that the obtained product was fairly clean (on a
water-free water basis the contaminant levels only slightly increase). With regard to the
specifications for ethanol fuel described by the Detroit Diesel Corporation [18], the
concentration of methanol, chloride ions and acetic acid were below the maximum limits.
Furthermore, higher alcohols, esters or furfural type of components were not traceable in the
ethanol distillate.
Anaerobic conversion of the liquid fraction to biogas
Directly after SSF and subsequent distillation, the distillation residue was collected and a
solid/liquid separation was performed by centrifugation. Using this method, 71.6 kg
distillation residue (17.3% dry matter and 9.7% insoluble solids) was divided into two
8
fractions: 52.7 kg (10.9% dry matter) liquid fraction serving as a substrate for biogas
production, and 18.6 kg (35.6% dry matter) of wet solid fraction functioning as a fuel for
thermal conversion tests. The centrifugation step, however, was not optimized resulting in a
liquid fraction containing a significant amount of solids present in the form of small particles
such as fines and salts. The liquid fraction, including dissolved organic compounds (for
example, xylose, arabinose, glycerol, acetic acid and lactic acid), was tested for potential
anaerobic biodegradability to biogas (methane). Figure 2A shows the changes in dissolved
chemical oxygen demand (COD), volatile fatty acids (VFAs) and methane production during
the anaerobic fermentation. Throughout the fermentation, no VFA accumulation or
acidification occurred (initial pH 6.7). The methane content of the biogas was 57% ± 5%.
Furthermore, Table 3 shows an anaerobic biodegradability of 57% ± 1% on a COD basis. The
remaining non-degraded fraction is most likely partly composed of non-biodegradable
dissolved COD (Figure 2A) and partly of non- or very slowly biodegradable solid matter, as
visual inspection after 8 days showed that the solid material present in the effluent had not
disappeared. Table 3 shows a COD balance of the experiment. It can be concluded from the
values that the dissolved fraction was almost completely degraded, whereas the non-dissolved
organic matter was partially degraded.
The effect of the potential presence of toxic components in the liquid fraction was tested by
performing an accumulation experiment. Figure 2B shows the methane production in the
accumulation test where the liquid fraction increased. No acidification took place and the
average methane content of the biogas was 53% ± 2%. Roughly the same amount of methane
was produced per feed, with an average degradability of 60% ± 1%. In conclusion, no
adaptation of the sludge or toxic effects of the liquid fraction were observed suggesting
feasible biodegradability of the liquid distillate residue.
9
Thermal conversion of the solid fraction
The centrifuged wet solid fraction (with 27% moisture) was thermally dried (to 3.4%
moisture) prior to laboratory-scale combustion tests. The composition of the solid fraction is
important for its thermal conversion behavior. The chemical analysis of the solid fraction,
together with the estimated composition corrected for 85% calcium recycle, original wheat
straw and typical values for clean, untreated wood are presented in Table 4. The ash content
of the experimental solid fraction was high (27.0% by weight) compared with straw (11.5%
by weight) and wood (1.5% by weight). Furthermore, the silica content of the solid fraction
increased significantly (by more than 4% by weight) compared with the wheat straw (2.8% by
weight), due to the removal and conversion of cellulose and hemi-cellulose during upstream
processes. As a result of the addition of calcium hydroxide and sulfuric acid during the
pretreatment process and the absence of any calcium removal for recycling purposes, the
concentrations of calcium (5.2% by weight) and sulfur (3.6% by weight) were increased
compared with wheat straw and high compared with woody biomass fuels. If a calcium
recycle is applied, these concentrations are estimated to be 0.8% by weight and 0.7% by
weight, respectively. The content of phosphorus (0.51% by weight) increased compared with
wheat straw owing to the addition of phosphorus containing additives during the fermentation
process. Relatively small amounts of potassium (0.30% by weight) and chlorine (0.15% by
weight) were found, more than in clean wood, but less than in the original straw as a result of
the washing effect of water in the upstream processes. The lower heating value (LHV) of the
LTWS solid fraction (experimental) was 12.4 MJ/kg (dry basis), which is lower in
comparison with the original wheat straw feedstock (15.6 MJ/kg) and wood (18.5 MJ/kg),
mainly because of the increased ash content. After mechanical dewatering of the solid
10
fraction, a realistic LHV of the solid fraction is estimated at 5 MJ/kg (wet basis) assuming
that 50% moisture in the fuel will remain for a process at industrial scale.
No single agglomeration index has been developed that reliably describes the agglomeration
behavior of a biomass fuel under all combustion conditions [19]. The agglomeration
indicators presented in Table 5 only give a rough indication and should be interpreted with
care. The indicators suggest a tendency for smelt-induced agglomeration for the fuels. The
indicators for fouling suggest that there is less risk of fouling.
During the combustion test, no signs of bed agglomeration were observed. As the experiment
lasted for only 4 hours, care should be taken to extrapolate this observation to the long-term
combustion behavior of the fuels. Longer tests have to be performed for a more profound
assessment. A scanning electron microscope was used to identify the possible build-up of a
coating layer on the bed material (sand), which can indicate possible agglomeration. No
potassium silica coating could be detected on the bed material after the experiments with
ethanol fermentation residues.
Owing to the higher sulfur content, sulfur dioxide (SO2) emissions will be a point of attention
and are significantly different from the regular combustion of wood, but well-known in coal
and straw-fired combustion installations. The measured hydrogen chloride (HCl)
concentration in the flue gas during the lime pre-treated residue combustion was
180/190 mg/Nm3 (at 6% volume O2) and the SO2 concentration was 3000/3360 mg/Nm
3 (at
6% volume O2). The SO2 concentrations are high as a result of the absence of a calcium
recycle during the upstream fermentation process in the pilot-scale fermentation tests. This
calcium recycle is, however, foreseen at the industrial scale. A large fraction of sulfur is
11
bound to the calcium present in the solid residue as gypsum and, therefore, not released into
the flue gas. With a calcium recycle, the SO2 flue gas concentration is expected to be 85%
lower, assuming the same ratio between sulfur and calcium (resulting in the same sulfur
capture ratio).
A deposition probe was placed in the flue gas for the duration of the whole test (4 hours).
After finishing the experiment, the probe was carefully removed and visually inspected.
Compared with wood combustion, the deposition rates of ash particles on the probe were high
due to the much higher ash content of the fermentation residues. The ash depositions could be
easily removed from the surface and appeared to be non-sticky. A facility for deposition
removal to keep the boiler tubes as clean as possible, such as a knocking mechanism, is
therefore recommended on an industrial scale, as is commonly used in, for example,
pulverized coal-fired power plants. No further analyses on the probe have been performed, as
no significant effects on the probe material are expected due to the limited duration of the
experiments.
Combustion in a circulating fluidized bed (CFB) was selected as the most suitable conversion
technology due to the small particle size of the lignin residue (less than a few millimeters),
moisture content (approximately 50% to 60% after dewatering) and the thermal capacity. The
combustion temperature in a CFB is relatively low (800 to 900°C) to reduce the risk of
sintering of the ashes in the bed. To reduce the risk of agglomeration, it is possible to partially
refresh the bed material during operation. It is expected that emission standards can be met in
a full-scale process by standard flue gas cleaning technologies (for example, flue gas
sulfurization, de-NOx, particle removal).
12
Overall balance
An overview of the lignocellulose-to-ethanol process with the respective process steps of side
streams is presented in Figure 3. Furthermore, the input and output streams were quantified.
With the results obtained by the SSF to ethanol, anaerobic digestion to methane and thermal
conversion, we calculated that 1000 kg of LTWS can be converted into 100 kg of ethanol
(equal to 3.0 GJ thermal energy calculated with a higher heating value (HHV) of 29.7 MJ/kg),
69 kg methane (equal to 3.8 GJ thermal energy calculated with an HHV of 55.5 MJ/kg),
approximately 204 kg sludge, 246 kg CO2 and 398 kg lignin-rich residue. The solid lignin-
rich fraction contains an HHV of 13.4 MJ/kg (dry basis) equal to a total of 5.3 GJ of thermal
energy for heat and power generation. Furthermore, the combustion of the lignin-rich fraction
results mainly in 458 kg CO2, 151 kg water and 107 kg inorganic ash.
Discussion
Lignocellulosic biomass has great potential with regard to its use as a substrate for the large-
scale production of biofuels. The lignocellulose-to-ethanol conversion via SSF is a complex
process involving various enzymatic and microbial activities. Results from previous studies
showed that the mild-temperature lime treatment (0.1 g Ca(OH)2 per gram substrate, 16 to 20
hours, at 85°C) enhances the accessibility of polymeric carbohydrates towards hydrolytic
enzymes resulting in the release of fermentable sugars and improved ethanol yield [14].
Nevertheless, these studies were performed at the laboratory and bench scale. This paper
describes the pretreatment, enzymatic hydrolysis and microbial conversion of LTWS via SSF
into ethanol on a pilot scale. Prior to using the lime-treated substrate for hydrolysis and
fermentation, the inhibitor acetic acid was successfully removed by a relatively simple
washing procedure.
13
Reaction conditions and enzyme loadings determined in previous optimization studies were
used for hydrolysis and fermentation. This resulted in a glucan-to-ethanol conversion (48%),
which is lower than the yields obtained in laboratory tests (56%) [14]. An explanation for this
difference in ethanol yield can be found in the higher dry-matter content, which was
approximately twice as high in the pilot-scale ethanol fermentation (around 20% by weight),
negatively influencing the enzymatic hydrolysis. A possible explanation for the low glucan-
to-ethanol yield can be found in the fact that xylose is present in the medium and may inhibit
the activity of the cellulolytic complex [20]. Another possible explanation for the lower yield
is that in this study the lime pretreatment was carried out on a pilot scale and without
continuous temperature control, thereby leading to lower cellulose digestibility compared
with a bench-scale test. Approximately 25% of the glucan in the LTWS remained present as
an insoluble polymeric sugar suggesting the presence of crystalline and hardly degradable
cellulose and/or inactivation of the hydrolytic system. Our findings support the results of
Chang et al [15] who reported that enzymatically hydrolyzed cellulose from lime-pretreated
biomass was utilized extensively by yeast, suggesting that the fermentations were not limited
by the glucose metabolism of the yeast.
The situation is different in the case of xylan. Owing to the inability of the wild-type baker’s
yeast to convert pentose sugars, end-product inhibition by xylose possibly occurs resulting in
the accumulation of mainly xylo-oligomeric sugars. Previous SSF work with LTWS,
hydrolytic enzymes and Bacillus coagulans, a pentose-converting, lactic-acid-producing
bacterium, showed that the concentration of xylo-oligomeric sugars and xylose remain low
[16]. This indicates that the presence of xylose monomers in the medium of the pilot-scale
ethanol process possibly inhibit the hydrolysis of soluble xylo-oligomeric sugars. In addition,
the xylan-to-xylose conversion at pilot-scale SSF (24%) agrees relatively well with the yield
14
obtained in laboratory-scale SSF experiments (28%). Further optimization of the enzymatic
hydrolysis and fermentation step can be realized by the use of pentose-converting yeast
resulting in a higher ethanol concentration and yield.
If a xylose-converting yeast is used, an additional amount of ethanol approximately 1.4 kg
(assuming the conversion of 74% of xylan, which is the fraction that was present as a
monomer and oligomer after 53 hours of incubation, with an ethanol yield of 0.51 g/g) can be
formed from this C5 sugar. Less dissolved substrate will be available for anaerobic digestion
resulting in an approximately 55% reduction of methane to around 0.5 kg.
Ethanol was upgraded via two separate distillation processes resulting in a distillate
containing 90% ethanol by weight. Tests showed that the ethanol distillate, in comparison
with the ethanol fuel specifications of the Detroit Diesel Corporation, seems suitable as a
transportation fuel.
The fermentation residue was separated into two fractions; one fraction contained mainly
solids functioning as a fuel for thermal conversion and the other was a liquid fraction that
contained mainly soluble compounds serving as a substrate for anaerobic fermentation to
biogas. The liquid fraction had a moderate anaerobic biodegradability of 60% respective to
the total COD content and no signs of toxicity could be observed in the accumulation test.
The relatively low value is largely a result of the high amount of solid organic matter in the
liquid fraction, which was only partly degraded. More efficient removal of the solid matter
would contribute to a significant reduction of the COD load. As the dissolved matter has a
high anaerobic biodegradability, anaerobic treatment would then be an attractive way of
processing this effluent for COD reduction and energy production in the form of methane gas.
15
The solid fraction of the distillation residue appeared to be suitable as a fuel for thermal
conversion, although it contained a relatively high ash concentration and relatively high
concentrations of sulfur and calcium due to additions in the upstream processes. It is expected
that the combustion will show a behavior between the original feedstock wheat straw and
(clean) wood. Based on the analyzed residue composition, there is some risk of smelt-induced
bed agglomeration. The risk of sinter-induced bed agglomeration is expected to be less than
for straw, as a result of the partial removal of potassium (washing) in the upstream processes.
The combustion experiments showed no agglomeration and no extensive coating was detected
on the bed material that would indicate agglomeration. Long-duration experiments on a larger
scale are necessary, however, to provide a more profound assessment of the fuel behavior.
With respect to fouling, high ash deposition rates were observed. The composition of both
bottom ashes and fly ashes were determined and it is most likely that both could be utilized
rather than being land-filled. The main options are utilization as building materials and, in
some cases, as fertilizers. A more conclusive assessment of the potential utilization of the
ashes requires larger samples that are more representative of the full scale, because the ashes
from the pilot-scale test likely have a lower salt content. Details on the assessment of ashes
will be presented in a follow-up paper.
Conclusions
This paper describes the successful up-scaling of the conversion of high dry-matter content
LTWS via SSF into bioethanol on a pilot scale. In comparison to conversions obtained in
laboratory experiments, we concluded that after 48 hours of incubation, slightly lower
fermentable sugar and ethanol yields were achieved at the pilot scale. Tests showed that the
16
produced ethanol was suitable as a transportation fuel. The side streams were quantitatively
analyzed. The liquid fraction containing mainly soluble components was valorized by
anaerobic fermentation to biogas (methane) and sludge. The remaining solid fraction was
assessed as a fuel for conversion by combustion yielding thermal energy and inorganic ashes
with the prospective of being utilized rather than being land-filled.
Methods
Lignocellulosic feedstock, enzymes and yeast
Wheat straw was purchased from a farm located in the northeast of The Netherlands. The
wheat straw was air-dried (89.5% dry matter by weight) and ground to pass through a 4-mm
screen. The chemical composition of the wheat straw was determined by using a modified
TAPPI Tcm-249 test method as described elsewhere [21]: glucan 33.3%; xylan 19.8%;
arabinan 2.2%; rhamnan 0.3%; mannan 1.1%; lignin 21.8%; uronic acids 2.5%; and
extractives 10.7%. The commercial enzyme cocktail GC 220 (Genencor-Danisco, Rochester,
NY, US) contained cellulase (filter paper units), cellobiase and xylanase activities of 116, 215
and 677 U/ml, respectively [22], with a specific gravity of 1.2 g/ml. The ethanol fermentation
was performed with the wild-type bakers’ yeast (Koningsgist, DSM, The Netherlands).
Chemicals, unless indicated otherwise, were purchased from Merck (Darmstadt, Germany).
Alkaline pretreatment and washing procedure
The alkaline pretreatment of air-dried wheat straw was performed in a 215-liter heated vessel.
About 22.5 kg ground wheat straw (approximately 20.1 liters per kilogram of dry matter) was
moistened with 40 liters of water and placed in the vessel. Subsequently, 2019 g of lime was
dissolved in 145 liters of water and added to the vessel and the resulting suspension was then
heated to 80–85°C under continuous mixing by a stirrer. After 8 hours of incubation (80–
17
85°C), the vessel was thermally isolated and throughout the following 16 hours of incubation,
the temperature of the wheat straw suspension decreased to 66°C. After a total of 24 hours of
incubation, the LTWS was transferred to a drainage vessel and dewatered through two sieves
(pore sizes 3 mm and 0.3 mm, respectively) by gravitation. The solid fraction was returned,
together with 215 liters of fresh hot water (55 to 60°C), into the drainage vessel, continuously
mixed for 30 minutes, and dewatered using the same approach outlined above. The
washing/dewatering procedure was repeated five times. The pH of the pulp was adjusted to
pH 6.5 with a 25% (by volume) sulfuric acid (H2SO4) solution. Finally, the pulp was
dewatered once more to a dry-matter content (105°C, 16 hours) of approximately 35% and
stored at 4°C.
For the determination of the insoluble solid content, the samples were centrifuged for
5 minutes at 3000 rpm, supernatant was removed and the remaining pellet was washed by re-
suspension with water. The sequence of washing and centrifugation was repeated three times
followed by drying the pellet overnight at 105°C.
Simultaneous saccharification and fermentation
The SSF of LTWS was carried out in a 100-liter fermenter (Applikon, Schiedam, The
Netherlands) with a pH and temperature control (Biocontroller ADI 1020). The reactor was
filled with 35 liters of tap water. The following salts were added to the water: ammonium
sulfate ((NH4)2SO4), 425 g; monopotassium phosphate (KH2PO4), 255 g; and magnesium
sulfate (Mg2SO4.7H2O), 42.5 g. The pH was adjusted and maintained at 5.0 with a 2 M H2SO4
solution and a 20% (weight to volume) Ca(OH)2 suspension. The process started with a
prehydrolysis phase (I) of 8 hours at 37°C and an agitation rate varying between 450 rpm and
700 rpm. During this phase, 48.42 kg of washed LTWS (with a dry-matter content of
18
approximately 35%) and 1105 ml GC 220 were added manually. The rate of substrate
addition depended on the viscosity of the LTWS suspension in the reactor. After
prehydrolysis, the following components were added to the LTWS: trace element solution,
85 ml; vitamin solution, 85 ml; fatty acid solution, 106.25 ml; anti-foam Acepol 77 (Emerald
Foam Control), 42.5 ml; antibiotics penicillin and steptomycin (Sigma-Aldrich), 212.5 ml;
second dosage of enzyme preparation GC 220, 787 ml; and fresh wet bakers’ yeast
(Koningsgist, DSM, The Netherlands), 1071 g (around 353 g dry yeast, 4.2 g dry yeast/liter
fermentation broth), all according to the methods described previously [14]. The SSF phase
(II) was initiated by the addition of yeast cells and the reactor was closed in order to analyze
CO2 production (Uras 10E, Hartmann & Braunn). A nitrogen gas flow between 5 and
40 liters/minute was added to the fermenter functioning as a carrier gas of the produced CO2.
As CO2 is produced in equimolar amounts with ethanol from glucose by bakers’ yeast,
measurements of CO2 production rates can be used to estimate the production rate of ethanol.
Samples were taken throughout the experiment, treated and analyzed for the composition of
monomeric sugars, organic acids, glycerol and ethanol by high-pressure liquid
chromatography methods as described previously [14]. The soluble oligomeric sugars and
insoluble polymeric sugars were determined as described previously [16].
Recovery of ethanol and quality assessment
After completion of the SSF process, the fermentation broth in the fermenter was heated up to
83°C. Nitrogen gas (15 liters/minute) functioned as a carrier gas of evaporated ethanol. The
ethanol was condensed by a cold fall spiral (length 270 cm, diameter 2 mm), which was
cooled by water at 4°C. The distillate fraction of condensed ethanol/water mixture was
collected in a bottle cooled with melting ice water.
19
Further upgrading of the ethanol content was performed in a second distillation step, which
was carried out in a batch distillation unit with a 3-liter boiler volume. An Oldershaw-type
distillation column containing 28 theoretical stages was used. The boiler was filled with 2 to
2.5 liters of the ethanol/water mixture obtained in the first distillation step. Hence, several
distillation runs were required to produce sufficient material. Distillate fractions of
approximately 20 g were collected separately and analyzed on ethanol content. During the
first phase of the distillation, the reflux ratio was kept at 2. After receiving the first 100 g, the
reflux ratio was increased to 3. This was done to ensure the required ethanol content of the
distillate (during a batch distillation the most volatile component is continuously depleted
from the liquid in the boiler). All distillate fractions were analyzed for their ethanol content by
gas liquid chromatography using a flame ionization detector (GLC-FID). The final distillate
sample (all combined distillate fractions with sufficient ethanol content) was analyzed for
water content (Karl Fischer titration, ASTM D 1364), acidity (acid-base titration, ASTM D
1613), anion content (chloride, sulphate, nitrate and carbonate measured by ion exchange
chromatography (ICE)) and organic components other than ethanol (gas liquid
chromatography-mass spectrometry (GLC-MS)).
Solid/liquid separation of distillation residue
After the distillation step, the distillation residue was separated by centrifugation (Heine
Zentrifuge, type DM-K-Z 2237-2V2, 1977), resulting in a liquid fraction and a fraction
containing mainly solid particles. The feeding rate of the distillation residue was around
6 liters/minute with an agitation rate of 3500 rpm.
20
Anaerobic biodegradation of the liquid fraction and biogas production
The anaerobic biodegradability of the liquid fraction to methane was studied by adding 20 ml
liquid fraction to 21.3 ml of macronutrients, trace elements and 10 mM potassium/sodium
phosphate buffer (pH 7) [23], inoculated with 75 g wet sludge. The mixture was adjusted with
water to 200 ml total volume and incubated at 30°C. The inoculum used was anaerobic
granular sludge from a reactor treating potato-processing wastewater with an organic matter
content of 0.08 g/g sludge. The sludge concentration was 30 g/liter. A blank without sludge
was used to correct for methane production from the inoculum itself. During the test, gas
production and composition, pH, COD and VFAs were measured. The dissolved COD
concentration was determined by the micromethod with dichromate (Dr Lange, Germany).
For determination of the VFA concentration, samples were withdrawn, centrifuged for
10 minutes at 10000 rpm and analyzed on a gas chromatograph (Hewlett Packard 5890
series II) with a 30 m Alltech column (AT-Aquawax-DA 0.32 mm × 0.25 mm). The
temperatures of the column, injection port and FID were 80°C (beginning) to 220°C (end),
275°C and 300°C, respectively. Nitrogen saturated with formic acid was used as the carrier
gas (20 ml/minute). The volume of produced biogas was determined using the Oxitop
pressure measurement system. The quantities of these components in the reactor gas were
determined on a gas chromatograph (Hewlett Packard 5890) with two different columns:
30 m Molsieve 5A 0.53 mm × 15 µm and 25 m Poraplot Q 0.53 mm × 20 µm. The carrier gas
was helium and the temperatures of the column, injection port and the thermal conductivity
detector were 45°C, 110°C and 99°C, respectively.
The potential toxicity of the liquid fraction was studied in an accumulation test with an
undiluted liquid fraction. The liquid fraction was added to the reactors in small quantities,
increasing the concentration with each addition until a final concentration of 80% liquid
21
fraction was reached with respect to the substrate/sludge relation (owing to time limitations,
reaching 100% was not possible). With each liquid fraction addition, macronutrients, trace
elements and phosphate buffer were also added to the system. The test was carried out at
30°C. The initial sludge concentration was 63 g organic matter per liter, after all additions this
was reduced to 15 g/liter. A blank without a liquid fraction was used to correct for methane
production from the inoculum itself. During the test, gas production and composition were
measured, as well as the pH, to check for acidification of the system.
Determination of the quality of the solid fraction for combustion
The solid fraction was assessed as a fuel for combustion. It was analyzed using standard
methods for the chemical analysis of biomass fuels according to the Dutch NEN NTA 8200
series. The results were used to calculate fluidized bed combustion agglomeration and fouling
indicators. Suitable conversion technology was selected using in-house expertise and data
from the literature [19, 24-26]. Combustion experiments were performed in a bench-scale
experimental FBC (feed rate approximately 0.15 kg/hour) [27]. The reactor had a diameter of
7 cm (bottom) and 11 cm (top) and a height of 110 cm and was made of 253 MA. Air was
used to fluidize the sand bed. The FBC was operated at a temperature of approximately 850°C
and 5% to 6% oxygen by volume in the dry flue gas. To prevent heat losses, the set-up was
electrically externally heated by tracing elements. The solid fraction was thermally dried
before combustion (approximately 3% moisture by weight) and milled (2 mm sieve diameter).
The fuel was transported into the reactor by a screw feeder. During combustion of the
fermentation residues, the fly ash was entrained with the flue gas and collected by Soxhlet
filters (200°C). The flue gas was analyzed online for: NOx, O2, CO2 and SO2, and off-line
(wet chemical) for SO2 and HCl. A deposition probe, cooled with water, was placed into the
22
flue gas to simulate boiling tubes in a boiler. After finishing the experiment, the probe was
carefully removed and visually inspected.
Competing interests
The authors declare that they have no competing interests.
List of abbreviations
CFB, circulating fluidized bed; COD, chemical oxygen demand; FBC, fluidized bed
combustor; FID, flame ionization detector; GLC-FID, gas liquid chromatography using a
flame ionization detector; GLC-MS, gas liquid chromatography-mass spectrometry; HHV,
higher heating value; ICE, ion exchange chromatography; LHV, lower heating value; LTWS,
lime-treated wheat straw, SSF, simultaneous saccharification and fermentation; VFA, volatile
fatty acid.
Authors’ contributions
RHWM carried out the SSF experiment and drafted the manuscript. RRB performed the
alkaline pretreatment and washing procedure and helped to draft the manuscript. ARB carried
out the thermal conversion of the solid fraction via combustion and helped to draft the
manuscript. IB carried out the anaerobic conversion of the liquid fraction and helped to draft
the manuscript. JRP performed the ash quality analysis and helped to draft the manuscript. EJ
participated in the design and coordination of the study and helped with the final discussion of
the manuscript. RAW participated in the design and coordination of the study and helped to
draft the manuscript. HR participated in the design and coordination of the study and helped
with the final discussion of the manuscript.
23
Acknowledgements
This research is supported with a grant from the Dutch Programme EET (Economy, Ecology,
Technology), a joint initiative of the Ministry of Economic Affairs, Education, Culture and
Sciences and the Ministry of Housing, Spatial Planning and the Environment. The Programme
is managed by the EET Programme Office, SenterNovem. The authors thank Mirjam A Kabel
and Jacinta C van der Putten for performing the carbohydrate analysis and Richard GM Op
den Kamp and Patrick FNM van Doeveren for providing technical assistance with the
pretreatment and fermentation.
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27
Figures
Figure 1 - Profiles throughout the simultaneous saccharification and fermentation of
lime-treated wheat straw by commercial enzyme preparation and Saccharomyces
cerevisiae at pH 5.0 and 37°C
(A) The formation of sugars. (B) The production of carbon dioxide. (C) The production of
fermentation products. The dotted line represents the addition of yeast and discriminates
between a prehydrolysis phase (I) and simultaneous saccharification and fermentation phase
(II). The error bars in the symbols denote the deviation of duplicate measurements.
Figure 2 - Changes in dissolved chemical oxygen demand, volatile fatty acids (VFAs) and
methane production during the anaerobic fermentation, and methane production during
the accumulation test
(A) Dissolved chemical oxygen demand, dissolved volatile fatty acids and methane
production during the anaerobic biodegradability test with a ten times diluted liquid fraction
(20 ml in a total of 200 ml). Data were corrected for the blank. (B) Methane production
during the accumulation test where the ratio of the liquid fraction to sludge was increased. All
data are represented as averages of duplicate experiments.
Figure 3 - Schematic depiction of an integrated process converting lignocellulose into
ethanol, biogas (methane), carbon dioxide, compost, energy (heat and power) and ash
The energy can be used for the process or delivered to the grid. The ‘clean’ water can be
recycled into several steps of the process.
28
Tables
Table 1 - Fate of the polysaccharide glucan and xylan initially present in washed lime-
treated wheat straw after 53 hours of simultaneous saccharification and fermentation
Fraction Glucan
(% by weight)
Xylanc
(% by weight)
Polysaccharides (insoluble)a
25 26
Oligosaccharides (soluble) 6 52
Hydrolysis
products
Monosaccharides (soluble) 1 24
Ethanol and carbon dioxide (soluble)
48 —
Fermentation
products Glycerol (soluble) 5 —
Unaccounted forb (Insoluble/soluble) 15 —
aA portion of the initial polysaccharides remained as insoluble polysaccharides.
bA portion of the initial polysaccharides were not recovered.
cXylan-derived sugars were not converted by yeast.
29
Table 2 - Analysis of the ethanol distillate and comparison with ethanol fuel
specifications [18]
Component Ethanol by fermentation Specificationsa
Ethanol
distillate
Test method used Values Test method
Ethanol 90.7% by weight
(=72.6% by
volume)
Gas liquid
chromatography
using a flame
ionization detector
(GLC-FID)/Gas
liquid
chromatography-
mass spectrometry
(GLC-MS)
92% by
volume
(minimum)
ASTM D 3545-90
Methanol 0.01% by weight GLC-FID/GLC-MS 2% by weight
(max)
ASTM D 4815-89c
Anion content
(for example,
chloride)
Less than
1 mg/liter
Ion exchange
chromatography
Chloride
0.0004%
weight to
weight
(maximum)
ASTM D 3120-87d
Water
9.6% by weight ASTM D 1364 0.5% by
weight
(maximum)
ASTM E 203-75
Acidity 0.0006% by
weight HAc
0.01 mg
potassium
hydroxide per
ASTM D 1613
HAc 0.007%
by weight
(maximum)
potassium
hydroxideb
ASTM D 1613-85
30
gram
aSpecifications for E-95 (denatured) ethanol fuel according to the Detroit Diesel Corporation.
bNo specifications available.
cOther alcohols and ethers.
dASTM D 3120-87 modified for determination of organic chlorides and ASTM D 2988-86.
Table 3 - Chemical oxygen demand balance for the degradability of the liquid fraction
(in grams of O2 based on a liter of effluent)
Chemical oxygen
demand (g O2)
Percentage of initial total
chemical oxygen demand
value
Initial state Total 134.9 100
Dissolved 54.6 40
Non-dissolveda 80.3 60
Final state Methane 77.2 57
Dissolved 9.4 7
Non-dissolveda 48.2 36
aNon-dissolved chemical oxygen demand was calculated from the measured values for total
and dissolved chemical oxygen demand.
31
Table 4 - Fuel analysis results of the lime-treated wheat straw solid fraction
(experimental and corrected for 85% calcium recycle) original wheat straw feedstock
and untreated wood
Source composition
Lime-treated
wheat straw
solid fraction
(experimental)
Lime-treated
wheat straw
solid fraction
(corrected for
85% calcium
recycle)
Wheat straw
original
feedstock
(harvest 2003)
Wood
(untreated,
average)
[28]
Parameter Unit
Ash (550°C) wt%dry 27.2 12.6 —
Ash (815°C) wt%dry 27.0 19.5 11.5 1.5
Volatiles wt%dry 68.4 75.5 70.1 80.7
Moisture wt%wet 3.4a 50
b 7.7 49.2
Higher heating
value, measured
MJ/kgdry 13.4 14.3 16.6 19.8
Lower heating
value
MJ/kgdry 12.4 13.2 15.6 18.5
Lower heating
value (at 50%
MC)
MJ/kgwet 5.0 5.4 6.6 8.0
C wt%daf 43.1 44.9 46.5 51
H wt%daf 5.8 6.1 5.6 6.1
N wt%daf 1.5 1.5 1.7 0.3
O wt%daf 44.8 46.6 45.8 43
S wt%daf 4.56 0.71 0.15 0.06
Cl wt%daf 0.20 0.21 0.31 0.04
F wt%daf 0.0031 0.0031 — 0.0024
32
Al mg/kgdry 121 133 185 211
As mg/kgdry 5.4 5.9 < dl less than dl
B mg/kgdry 5.2 5.7 4.1 9
Ba mg/kgdry 3.2 3.5 3.6 143
Ca mg/kgdry 52000 8600 4700 13000
Cd mg/kgdry Less than dl — 0.15 0.5
Co mg/kgdry 7.3 8.1 55 0.51
Cr mg/kgdry 10.2 11.2 0.2 5
Cu mg/kgdry 9.1 10.1 1.6 17
Fe mg/kgdry 160 170 220 120
K mg/kgdry 3000 3300 15000 2000
Li mg/kgdry Less than dl — 0.2 Less than dl
Mg mg/kgdry 870 36 1105 769
Mn mg/kgdry 33 36 14 214
Mo mg/kgdry Less than dl — 1.9 0.55
Na mg/kgdry 1500 1660 200 300
Ni mg/kgdry 5 5.8 0 8
P mg/kgdry 5100 5600 485 486
Pb mg/kgdry Less than dl — 1.1 32
Sb mg/kgdry 1.4 1.6 1.8 1.8
Se mg/kgdry Less than dl — 1.9 1
Si mg/kgdry 40000 44500 28000 900
Sn mg/kgdry 0 0 0.20 1.2
Sr mg/kgdry 23 25 18 14
V mg/kgdry 0.3 0.3 — —
Zn mg/kgdry 25 27 0.6 0.4
aAfter thermal drying.
bAfter mechanical dewatering with Pneumapress.
33
Table 5 - Agglomeration and fouling indicators of the lime-treated wheat straw solid
fraction (experimental and corrected for 85% calcium recycle) original wheat straw
feedstock and untreated wood
Source composition
Lime-treated
wheat straw
solid fraction
(experimental)
Lime-treated
wheat straw
solid fraction
(corrected for
85% calcium
recycle)
Wheat straw
original
feedstock
(harvest 2003)
Wood
(untreated,
average)
[28]
Fluidized bed combustion agglomeration indicatorsa
Sinter-inducedb
(1) Na + K/2S + Cl
more than 1
0.1 0.4 2.7 1.4
(2) (Na + K + Si)/
(Ca + P + Mg) more
than 2
0.8 3 7.0 0.2
Smelt-inducedc
(3a) K + Na/Si more
than 0.6
0.1 0.1 0.5 2.6
(3b) K + Na more than
1 g/kg
5 5 15 2.3
(3c) Si more than 1.5
g/kg
40 44 28 0.9
(3d) K2O + Na2O +
SiO2 more than 50%
ash
34 52 68 30
Fluidized bed combustion fouling indicators (at 50% moisture content)a
34
3 Cl +S more than 1%
weightar
2.1 0.6 0.9 0.2
Na + K + Zn more than
1% weightar
0.2 0.3 0.4 0.2
Dust fraction of ash more
than 30%
Unknown Unknown Unknown
Alkali/DOE
indexd
kg/GJ
HHVe
0.3 — 0.9 0.1
aVisser and Laughlin [29].
bThe second indicator is relevant if the first indicator is exceeded.
cThe more the indicators exceed the limit value, the more likely it is that smelt bed
agglomeration will occur.
dMiles et al [30]: if the value of the index is higher than 0.17, probably fouling; if more than
0.34, certainly fouling.
eHigher heating value.
0
200
400
600
800
1000
0 10 20 30 40 50
Time (h)
CO
2 p
roductio
n (
l)
I IIB
0
5
10
15
20
25
0 10 20 30 40 50
Time (h)
Concentr
atio
n (
g/l)
II
A
I
0
5
10
15
20
25
0 10 20 30 40 50
Time (h)
Concentr
atio
n (
g/l)
C
I II
Key
A) glucose(�), xylose (ゴ) and arabinose (�)
C) ethanol (メ), glycerol (ミ) and lactic acid (�). Figure 1
0
1
2
3
4
5
6
7
8
0 1 2 3 4 5 6 7 8 9
Time (d)
CO
D (
g/l)
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
Meth
ane p
roductio
n (
l)
A
0
1
2
3
4
5
6
0 10 20 30 40 50
Time (d)
Meth
ane p
roductio
n (
l)
0
0,2
0,4
0,6
0,8
Ratio
liquid
fractio
n/s
ludge (
g/g
)
B
Key
a) Dissolved chemical oxygen demand (許), dissolved volatile fatty acids (汲) and
methane production (line)
b) Methane production (line), ratio of the liquid fraction to sludge (虚)
Figure 2
Figure 3